Greenies recommends owners check that the treats are chewed and Joe Roetheli - who launched the brand as a treat that can freshen a dog's breath and clean its teeth - said it was important to pick the correct chew for a particular dog. There are 7 different sizes to choose from depending on the size of the dog.

But most of the dog owners CNN talked to say they did follow package instructions and they still had a problem.

Mike Eastwood and his wife, Jenny Reiff, recently filed a $5 million lawsuit in New York, blaming Greenies for the intestinal blockage that caused the death of their dog Burt.

"I'm mad that their packaging states that the product is 100 percent edible, highly digestible and veterinarian approved, yet our dog died of it," Eastwood told CNN.

S&M NuTec, which manufactures the toothbrush-shaped chew, won't comment on the case but in court papers denied the allegations.

Roetheli said the focus should be on the dental benefits and Greenies are saving dogs' lives by lowering the risk of periodontal disease.

He says feeding Greenies is far safer than putting a dog under anesthesia to clean teeth.

"Dogs really love the product!" he said. "They do a very effective job of cleaning teeth and freshening breath."

Any suggestion that Greenies are defective was rejected by Roetheli, who developed Greenies with his wife, Judy.

"Our product is safe. It is used every day by thousands of dogs, millions a week and it is basically a very safe product."

A CNN investigation uncovered 40 cases since 2003 where a veterinarian had to extract a Greenie from a dog after the treat became lodged either in the animal's esophagus or intestine. In 13 of those cases, the pet died.

One of those was Tyson, Josh Glass and Leah Falls' 8-month-old boxer, who was taken to Brent-Air Animal Hospital in Los Angeles, California, where vet Dr. Kevin Schlanger found the animal had a blocked intestine.

"It was very clear that it was something dense and firm that had caused the obstruction," Schlanger said. He removed a Greenie from the intestine.

McKiernan's says his Denver clinic has seen at least seven cases in the past five years, which he says is an unusually high number. That prompted him to start researching and writing a paper to warn other veterinarians of the problem.

He says his research, which he hopes to get published in a veterinary journal, shows compressed vegetable chew treats, of which Greenies is the most popular, are now the third biggest cause of esophageal obstruction in dogs behind bones and fish hooks.

The federal Food and Drug Administration says it's looking into eight consumer complaints about Greenies but has no formal investigation.

The issue has also been the topic of news reports across the country.

The chews are made of digestible products like wheat gluten and fiber, experts say, but the molding process makes the treat very firm and hard.

Roetheli, who runs S&M NuTec from Kansas City, Missouri, says Greenies do break down when properly chewed and swallowed by a dog.

He told CNN that any product has the potential to cause an obstruction in a dog and that Greenies packaging warns dog owners to monitor their dog to ensure the treat is adequately chewed. "Gulping any item can be harmful or even fatal to a dog," the package says.

The company's Web site addresses the issue in its FAQ section with the question "When giving an animal Greenies, does it affect their digestive system?" The answer "The only time dogs would be unable to digest anything would be if they didn't chew it up before they swallowed it. Canine and Feline Greenies are highly digestible when chewed."

The company says the number of complaints it has received is very low in relation to the vast numbers of treats sold, and CNN spoke with several vets who recommended Greenies.

Introduced in 1998, we found Greenies now selling for about $16 a pound. Last year, 325 million individual treats were sold around the world, nearly three times the sales of its nearest competitor Milk Bone, according to the marketing company Euromonitor International.

"At the end of the day ... literally millions of Greenies are enjoyed by dogs on a weekly basis with absolutely no incidents," company vet Brad Quest told CNN.